1. Field of the Invention
The present invention relates to a method and apparatus for reproducing data from an optical disc and, more particularly, to a method and apparatus for compensating audio signals reproduced from the optical disc.
2. Description of the Prior Art
A Compact Disc (referred to as “CD” hereinafter) is a conventional recording medium which records data digitally. Because data is recorded digitally, it does not deteriorate when reproduced, even if the CD is used repeatedly. However, the CD, which is presently used throughout audio and computer fields, has limited recording capacity that restricts its use in video. A Digital Versatile Disc (“DVD”) has been recently developed as a new recording medium suitable for the multi-media age in the recording media market. The DVD is able to store moving images as well as numbers, characters, figures and voices. The DVD has all the advantages of the CD, and has a recording capacity of about 5.2 Gbytes per side. Therefore, a complete conventional movie, including moving images, can be sufficiently recorded on one DVD.
Physically, the DVD is as small and durable as a conventional CD. Furthermore, data stored on the DVD is recorded digitally, rendering it able to be preserved easily. For these reasons, the DVD is an alternative recording medium that has become widely used in the market of recording media including video/audio and computer fields. The wide use of the DVD in image fields has given the DVD a good reputation as an image recording media.
Conventional reproduction apparatuses for reproducing video/audio signals from the DVD, and the operation thereof, are described in detail below with reference to FIGS. 1 to 4C of the attached drawings.
FIG. 1 is a block diagram showing a conventional reproduction apparatus for the optical disc. As shown in FIG. 1, the conventional reproduction apparatus comprises: an optical disc (i.e. DVD) 1 on which video/audio signal data are recorded; an optical pick-up apparatus 3 for reading the data recorded on the optical disc 1; and a motor 11 for rotating the optical disc 1; and a servo-circuit 13 for controlling the motor 11 and the optical pick-up apparatus 3.
The conventional reproduction apparatus further comprises: a micro-processor 15 for managing the overall control of the reproduction apparatus according to a user's request, and for controlling the servo-circuit 13; a navigator 17 for receiving commands from the micro-processor 15, and for executing the commands as to the transmission of data; and a high frequency amplifying circuit 5 for amplifying data read from the optical pick-up apparatus 3 in high frequency bands, and for outputting amplified signals under the control of the navigator 17.
The conventional reproduction apparatus also comprises: an error correcting circuit (ECC) 7 for correcting errors in bit stream of amplified signals output from the high frequency amplifying circuit 5 and for outputting corrected signals under the control of the navigator 17; and a Variable Bit Rate buffer (VBR buffer) 9 for temporarily storing signals output from the error correcting circuit 7 under the control of the navigator 17. The VBR buffer 9 may be a First In First Out (FIFO) buffer.
In addition, the conventional reproduction apparatus comprises a data decoding unit 20 which is composed of a video decoding circuit 21, a graphics circuit 25 and an audio decoding circuit 27. When the bit stream output from the VBR buffer 9 is input to the navigator 17 and the data decoding circuit unit 20, the video decoding circuit 21 extracts only video signal data therefrom and decodes the video signal data based on data dividing control signals of the navigator 17. Similarly, the graphics circuit 25 extracts and decodes only caption signal data, and the audio decoding circuit 27 extracts and decodes only audio signal data.
The video data decoded by the video decoding circuit 21 and the caption signal data decoded by the graphics circuit 25 are mixed in a mixer (not shown in the attached drawings), and then the mixed data is converted into analog signals by a video digital/analog converter 23, which is displayed after being adjusted into broadcasting signals in a NTSC/PAL encoder 31. In the similar manner, the audio data decoded by the audio decoding circuit 27 is converted into analog signals by a audio digital/analog converter 29, which audio data is then synchronized with the video signals and output.
The operation of the conventional reproduction apparatus of the optical disc will be described in the following.
The video/audio data recorded in the DVD is composed of user data and system data. The user data is composed of video data formatted in VBR format which will be processed in the video decoding circuit 21, and audio data which will be processed in the audio decoding circuit 27. The system data is composed of information relating to systematic functions, by which, for example, the video/audio data selected by the user are read from the DVD and transmitted to the audio decoding circuit 27 and video decoding circuit 21 at a suitable rate.
FIG. 2 shows a conventional DVD format. As shown in FIG. 2, the user data recorded in the DVD 1 is composed of the video data and the audio data. By comparison, the recording capacities of the video data and the audio data are arranged such that about 9 frames of the video data are recorded for every one frame (1536 bytes) of audio data. The disparity between audio and video data stored on the disc is a consequence of the disproportionate size of audio and video frames. The video data includes, for example, data corresponding to moving images which generally occupies much more space to than the audio data.
When reproduced from the DVD, the video data and the audio data are adequately amplified. The amplified signals being corrected in the error correcting circuit 7 before being temporarily stored in the VBR buffer 9.
The signals output from the VBR buffer 9 are input into the data decoding unit 20 under the control of the navigator 17, and the data dividing operation is executed in at least one of the decoding circuits of data decoding unit 20. Then, the divided signals are decoded, respectively, by the following process.
FIG. 3A is a detailed circuit diagram of the video decoding circuit 21 illustrated in FIG. 1.
The video decoding circuit 21 comprises a parser 33, a video decoding unit 35 and a memory 37. The parser 33 receives the bit stream from the VBR buffer 9 via a first input terminal and the data dividing control signals from the navigator 17 via a second input terminal. The parser 33 extracts only the video data in accordance with the data dividing control signals, and outputs the video data to the video decoding unit 35. Other video-related signals will be parsed to subsequent stages (such as, the graphics circuit 25 and audio decoding circuit 27). The video decoding unit 35 generates original signals by decoding the video data extracted in the parser 33. The video decoding unit 35 may also temporarily store the decoded data of the original signal in the memory 37, and output the original signals stored in the memory 37. The video decoding unit comprises a control unit (not shown in the attached drawings) for monitoring the storage volume of the data stored in the memory 37, and for outputting a data transmitting request signal to the navigator 17 based on the storage volume of the data stored in memory 37.
In other words the video decoding circuit 21 decodes the video data input from the parser 33, temporarily stores the decoded video data in the memory 37, and outputs signals based on stored data when the data corresponding to a predetermined screen portion are stored. The video decoding circuit 21 also monitors the storage volume of the data stored in the memory 37, in which the data decoded in the video decoding unit 35 are stored. So long as there is memory space in the memory 37, the video decoding unit 35 will decode additional video data by outputting the data transmitting request signal to the navigator 17. However, if the memory 37 is full, the video decoding unit 35 adjusts the input of video data to memory 37 by outputting a data transmitting stopping signal to the navigator 17.
FIG. 3B is the detailed circuit diagram of the audio decoding circuit 27 included in the conventional device illustrated in FIG. 1. The audio decoding circuit 27 comprises a parser 39, an audio decoding unit 41 and a memory 43. The parser 39 receives the bit stream from the video decoding circuit 21 via a first input terminal, and receives the data dividing control signal from the navigator 17 via a second input terminal. The parser extracts only the audio data in accordance with the data dividing control signals. Thereafter, the audio decoding unit 41 decodes the audio data extracted by the parser 39, and temporarily stores the audio data decoded in the memory 43. In other words, the audio decoding circuit 27 decodes the audio data input via the parser 39, temporarily stores the decoded data in the memory 43, and continuously outputs the data stored in the memory 43.
Unlike the video decoding circuit 21, the audio decoding circuit 27 docs not have any means to output a data transmitting request signal or data transmitting stopping signal to the navigator 17. That is, in the conventional reproduction apparatus for the optical disc, the rate at which the data is input into the audio decoding circuit 27 is adjusted only according to the storage volume of the data stored in the memory 37 of the video decoding circuit 21 used to decode the video data.
As mentioned earlier, the size of recorded video data is about 9 times that of recorded audio data, as shown in FIG. 2. The memories 37 and 43 are therefore pre-set so that the ratio of the space of the memory 37 in the video decoding circuit 21 and that of the memory 43 in the audio decoding circuit 27 is also about 9:1. However, audio data is irregularly recorded at various positions on the DVD, and the decoding operation of the video decoding circuit 21 is not always synchronized with the decoding operation of the audio decoding circuit 27. Therefore, if the audio signal data is input into the audio decoding circuit 27 depending on the video decoding circuit 21, audio signals outputted can become discontinuous.
In order to overcome the above problem, the memories 37 and 43 may be designed so that the ratio of recording space in the memory 37 of the video decoding circuit 21 to the recording, space in the memory 43 of the audio decoding circuit 27 is less than 9:1. To achieve this reduced proportion without sacrificing video data memory capacity, the memory 43 of the audio decoding circuit 27 must be made larger. However, in such case, the cost of the apparatus rises and the memory 43 can not be used efficiently.
In addition, since the audio decoding circuit 27 can not generate the data transmitting request and stopping signals independently, the conventional apparatus has another problem in that overflow and/or underflow of the data occurs in the memory 43 of the audio decoding circuit 27.
As shown in FIG. 4A, the length of one frame of the normal audio data being input into the audio decoding circuit 27 is 1536 bytes. The audio decoding circuit 27 in the conventional reproduction apparatus, as shown in FIG. 2, therefore processes the audio data one frame (1536 bytes) at a time. More specifically, before recording the data in the DVD, audio signals are processed (e.g. encoding the audio data or inserting the error correction code in the audio data, etc.) in increments of 1536 bytes. Therefore, the audio decoding circuit 27 also has to perform the data processing operation per 1536 bytes.
However, as shown in FIGS. 4B and 4C, the length of the audio data read from the optical pick-up apparatus 3 may be larger or smaller than the normal length (1536 bytes) due to a disc error or a recording error. In these situations, a further problem will occur since the audio decoding circuit 27 does not decode the audio signal data normally, causing error signals to be generated as described hereinafter.
If the size of the bit stream of the audio data being input into the audio decoding circuit 27 is smaller than 1536 bytes (refer to FIG. 4B), some data of the audio data bit stream of a subsequent frame will be processed together with the data of the present frame, causing errors to be generated. Also, if the bit stream input is larger than 1536 bytes (refer to FIG. 4C), the remaining data after the 1536th byte in the present frame will be processed with the data of the subsequent frame. In both cases, the decoding operation of the audio data can not be performed normally.